Method of depositing copper and baths for use therein



proximate 5.0 electrometric.

Patented Aug. 28,1945

METHOD OF DEPOSITING COPPER AND BATES FOR USE THEREIN Jesse a. smack,Oakville, and Frank lassalacqua, Waterbury, Conn.,

now by Judicial change of name, Frank Passal, assignors to UnitedChromium, Incorporated, New'York, N. Y., acorporatlon of Delaware NoDrawing. Application May 1, 1941, Serial No. 391,310

4 Claims. (Cl. 204- 52) This invention relates to the electrodepositionof copper, to electrolytic solutions for use there-- in, and providesimprovements therein.

In the -art of electroplating, it is a desideratum to be able toelectrodeposit a copper coating as smooth, fine-grained and ductile aspossible and with as high a tensile strength as possible, within theshortest time, i. e., at the highest current density. These coatings ofcopper, which may range in thickness from a few ten-thousandths of aninch to several thousandths, should have structural characteristicswhich permit easy buffing, polishing. inding, turning and etchingoperations. The surface obtained by these operations should beessentially free of surface imperfections such as pits, non-uniformityof grain structure, etc. so as to make these coatings useful andsatisfactory as a basis for subsequent coatings such as nickel,chromium, etc. and for use as printing forms, imprinting or embossingdies, building parts to size, etc. Processes looking toward these endsare known in the prior art, notably the difierent modifications of thecopper sulfate-sulfuric acid bath which have long been used in the artof electroplating.

The present invention provides a'copper plating bath having as itsimportant and primary constituents the bivalent copper, the ammoniumand'the formate radicals, and adjusted to a pH value between 2.0 and 4.0electrometric. It also provides a method of electrodepositing dense,fine-grained, and smooth deposits of copper, of high tensile strength,throughout a wide current density range. It also provides a method' ofregulating and maintaining the bath for continuous operation.

We have found that a. copper salt such as copper sulfate, copperformate, etc. may be dissolved in water and sufiicient ammonium andformate added in the form of ammonium hydroxide and formic acid, or inthe form of salts containing these radicals, either combined orseparately, such as ammonium formate or ammonium sulfate and sodiumformate, to give a clear dark-blue solution at pH values to ap- From theincrease in the permissible total salt concentration and. from thedeepeningof color-which occurs on the addition of an ammonium-containingcompound to a formate-copper solution, it is believed that a complex ofthe double salt type exists in our solution. By reason of this doublesalt complex, the high ionic mobility of the formate radical and thehigh buflering effect obtained, deposition of smooth, fine-grainedcopper coatings on a variety of basis metal surfaces which have beensuitably treated is obtained at high current densities. Certainnon-ferrous metals such as brass andbronze may be plated directly, aftera suitable cleaning cycle, with good adherence of the copper coating tothe basis metal. With some other metals, such as nickel it is oftennecessary to activate them as by gassing cathodically in a solution ofsulfuric acid before electrodepositing the copper, in order to obtaingood adherence. With other metals, such as steel, cast iron, zinc-basedie castings, etc. which have a tendency to displace metallic cop perfrom a bivalent solution in the acid pH range and form a looselyadherent film of copper, a fiash" or strike" in an alkaline coppersolution, such as a cyanide copper or a pyrophosphate copper bath isapplied, before electrodepositing copper from the present bath forobtaining good adherence. The. application of an adherent andcontinuous. copper coating to the basis metal surface in a suitablealkaline copper bath prior to immersing the surface to be plated in theacid copper bath will result in adherent coatings.

Specific examples of the preparation of two 'baths usable in our processare the following:

In preparing this solution the stated amount of copper sulfate wasdissolved in 500 cc. of water. The stated volume offormic acid wasvolume of ammonium hydroxide. The resulting solution was then diluted toa volume of one liter with water, and the pH adjusted as hereinafterdescribed. 5

EXAHPLI'Z Solution composition Copper formate (technical grade 32.4% cu)(47.1% formate) g./l 185 Ammonium formate (NH4OOC.H) g./l 87.5

in which the concentration of copper, formate and ammonium is asfollows:

Copper (Cu) -i g./l so Formate (H.000) g./l.. 155 Ammonium (NH4) g./l

In preparing this solution, the stated amount of copper formate wasdissolved in 750 cc. of water. To the resulting solution, the statedamount of ammonium formate was then added and the solution was stirreduntil this salt dissolved. The resulting solution was then diluted to avolume of one liter with water, and the pH 25 adjusted as hereinafterdescribed.

The two specific examples of solution preparaiion cited representsolutions containing the primary constituents. Other constituents suchas boric acid and borates, phenolsulfonic acid, gum arabic, gelatin,glue, dextrin, molasses, starch, casein, alum etc. may be added asbrighteners or buffering agents.

In commercial operation of the bath, the pH is adjusted to a value ofbelow 4.0 electrometric with a preferred optimum operating range of 2.5to 3.5 electrometric. After the bath has been prepared in a mannersimilar to that cited in Examples #1 and #2, the pH of the resultingsolution will have a value approximately within a range of 2.0-4.5 asdetermined with a glass electrode. If the pH is below the preferredrange of 2.5 to 3.5, it-may be raised so as to come within this range byan addition of a basic compound, 1 such as copper carbonate, sodiumhydroxide, sodium carbonate, ammonium hydroxide, etc. If th pH of theprepared bath is above the preferred range of 2.5 to 3.5, it may belowered to a value within this range by the addition of a small volumeof a concentrated sulfuric acid solution or of a formic acid solution.

During the commercial operation of the bath, the pH will-have a tendencyto slowly drift either upward or downward, the rate of which will dependon such factors as bath temperature, degree, of agitation, rate of anodecorrosion, anode and athode current densities, etc. If the pH of thebath is too low (below 2.0), the tendency will be to increase loss byvolatilization of formate as formic acid and the formation ofcrystalline films on the anode. If the pH is too high (above 4.0), thetendency will be to co-deposition of cuprous cxide and basic salts ofcopper with metallic copper at the cathode, and also to the formation ofa non-conducting film at the anodes.- To obtain a satisfactory-andpractical working of the bath, and to avoid the consequences ofthe abovestated tendencies, the pH thereof should be regulated and maintainedbetween values of 2.0 and 4.0 as measured-with a glass electrode with apreferred optimum range of 2.5 to 3.5. Th relative concentrations of thebath constituents may be varied widely and the solution will still havesatisfactory plating characteristics. However, we generally prefer ametallic copper I then added, followed by the addition of the statedconcentration of 45 to 75 g./l., a'formate concentration of 75 to 155g./l., and an ammonium concentration of 20 to 30 g./l. The copper may beconveniently added as copper sulfate, the formate as formic acid, andthe ammonium as aqua ammonia solution. The concentrations of copper,formate and ammonium may be suitably adjusted to meet requirements for aparticular operating procedure such as current density range desired,degree of agitation available, bath temperature, etc. Baths ofexceptionally high copper concentration are obtainable by the presentinvention. Commercial cyanide copper baths have a copper concentrationof about 22 to 30 g./l. (3 to 4 oz./gal.) acid copper baths of about 45to 52 g./l. (6 to 7 oz./gal.); whereas commercial baths according to thepresent invention may well have 60 to '75 g./l. (5 to 10 oz./gal.).

The primary constituents of the bath herein disclosed are eachchemically stable and will not appreciably decompose on being subjectedto electrolysis. On electrolysis at elevated temperatures, some loss offormate by volatilization as formic acid will occur, resulting in a slowrise in pH. This necessitates periodic additions of ftrmlc acid toreplace the loss of formate. Since the bath is extremely well buffered,the operating pH range is relatively wide, and the allowable formateradical concentration has relatively wide limits, the slow loss offormic acid will not present any serious problems in productionoperation of the bath.

' Under ordinary operating conditions, temperatures ranging from to F.or higher may be satisfactorily used, the particular temperature beingchosen with reference to the speed of deposition desired, and otherconsiderations usual with a plater. An increase in temperature willresult in a corresponding increase in the maximum current densityavailable. It is recommended, however, that the operating temperature bekept as low as production requirements permit so as to minimize the lossof formic acid by volatilization.

j Operating current densities of around 300 amperes per square foot arepracticable, depending .on such factors as solution composition,operatplastic, acid-proof brick, etc. Copper anodes are Cu g./1 63Formate g./l NR4 i g.'/1 24 pH (electrometric) 2.70

using a cathode current density. of 260 amperes per square foot and abath temperature of 95 F.

The deposit thus obtained was suitable for rotog'ravure printing, beingeasily polished and etched; I

From the foregoing it will be perceived that there is provided a bathand a method by which copper deposits of good grain structure or textureare obtained at a high rate of deposition,

higher in fact for comparable quality of deposit than any previouslyknown process.

What is claimed is:

1. An aqueous bath for electrodepositing cop-.

per, comprising essentially, in solution, 45 to 75 g./1. of copper, 75to 155 g./l. of formate radicals and 20'to 30 g./l. ammonium radicals,and having a pH value between 2 and 4 electrometric.

2. An aqueous bath for electrodepositing copper, according to. claim 1,wherein said pH value is between 2.5 and 3.5 electrometrlc.

. cording to claim 3, wherein the pH value is maintained between 2.5 and3.5.

JESSE E. STARECK. FRANK PASSALACQUA.

